1. Field of the Invention
This invention relates to an apochromat type objective lenses used as a lens for an astronomical telescope, a photographic telephoto lens and the like.
2. Description of the Prior Art
In the past, as an achromatic lens for a telephoto lens, an achromat wherein two lenses made of two kinds of optical glasses are combined, or an apochromat wherein three or more kinds of optical glasses are used, and a secondary spectrum in achromat is removed so that focal points of line C (.lambda.=6363 .ANG.), line D (.lambda.=5893 .ANG.) and line F (.lambda.=4861 .ANG.) are coincided have been used. The secondary spectrum in achromat termed herein is a difference of focal points between line C, F and line D. The line C, line D and line F are a rift in solar spectrum, which is a Fraunhofer dark line. These dark lines are present in orange color, yellow and dark blue. The aforesaid apochromat is much smaller in residual chromatic aberration i.e. secondary spectrum than the aforesaid achromat and excellent in resolving power, while it is extremely difficult to manufacture and is expensive.
In view of the foregoing, recently, there has been proposed that a low-dispersion optical glass or a new kind be used to produce a lens which has a performance close to the apochromat in terms of the residual chromatic aberration while it is an achromat in terms of construction, which lens is called a semiapochromat lens. The lens of this kind has its residual chromatic aberration which is about one-several of an ordinary achromat, and can be used to produce a telephoto lens of good quality up to an aperture about 10 cm of an aperture. FIG. 1 shows a typical trend of residual chromatic aberrations of the aforementioned various lenses.
Since the conventional apochromat is constructed as described above, it is very difficult to manufacture a lens which is small in residual chromatic aberration and excellent in resolving power by three kinds of optical glasses and in addition, it is very expensive.
Moreover, in the case of an aforesaid semiapochromat lens, and it is strongly required that the larger the aperture and the smaller the aperture ratio is, the smaller the residual chromatic aberration must be. Therefore, it is extremely difficult to produce a telephoto lens of large-aperture, compact size and high performance.
That is, in the semiapochromat lens, the maximum available aperture is about 10 cm for a focal length of 1 m. If the aperture is set to 15 cm, the focal length should be 2 m or more in order to provide a lens of good quality.
Accordingly, as described above, in the semiapochromat lens using a low-dispersion optical glass, there was a problem in that a compact and practical high performance telephoto lens having an aperture more than 15 cm cannot be obtained.